1. Field of the Invention
This invention relates to a lens system for a television image projection apparatus, and more particularly to a lens having an F number of 1.0 or less and a field angle of about 40.degree. for projecting on a screen an enlargement of an image displayed on a cathode ray tube.
2. Description of the Prior Art
For obtaining a television image on a large picture screen, there is a method of enlarging and projecting by a lens onto the screen the image displayed on the cathode ray tube. In this case, it is desirable that the projected image on the screen be as bright as possible so that the cathode ray tube should emit light as high as possible and also a bright lens should be used. For the purpose of the above, there have been known bright projection lens systems with F 1.0 consisting of six glass elements. Also U.S. Pat. Nos. 4,300,817 and 4,348,081 disclose projection lens systems each of which consists of three elements made of acrylic resin and which have an aspherical surface. In a case of using lenses all of which are made of acrylic resin, since the temperature dependency of the refractive index of acrylic resin is larger by one digit than that of glass, the image moves due to an ambient temperature change, so that a focal shift or a misconvergence occurs. Therefore, a hybrid lens consisting of three elements made of glass and acrylic resin has hitherto been proposed. A further bright lens of F1.0 or less is desired, but this will create the following new problems.
The optical system at present mainly used for a television image projection apparatus, as shown in FIG. 1, uses three cathode ray tubes 1R, 1G and 1B which are red, green and blue respectively and three lenses 2R, 2G and 2B. The three lenses 2R, 2G and 2B are disposed in the same plane so that the optical axes 3R, 3G and 3B converge at one point on a screen 4 or in the vicinity thereof. The green cathode ray tube 1G is disposed behind the central lens 2G and on the common optical axis therewith, but the red and blue cathode ray tubes 1R and 1B must be tilted with respect to the optical axes 3R and 3B of lenses 2R and 2B respectively in order to register the projected image planes. The tilting angle .theta. becomes larger as the F number of each lens 2R, 2G and 2B becomes smaller. In order to make the apparatus compact, when the projection distance (the distance from the utmost end of lens 2G to the screen 4) is reduced, the tilting angle .theta. becomes larger. Thus, when the tilting angle .theta. becomes larger, the distance between the lens and the cathode ray tube (distance between the lens 2G and the cathode ray tube 1G) should be increased as required. In a case where the heat developed in the cathode ray tubes 1R, 1G and 1B adversely affects the lenses 2R, 2G and 2B respectively, the distance between the lens system and the cathode ray tube should be further increased. In the aforesaid conventional example, the distance between the lens system and the cathode ray tube has been insufficient.